Image printbars which are used in image recording systems are well known in the art. Such printbars are generally comprised of a linear array of a plurality of discrete, light-emitting sources. Examples of such printbars include wire dot, electrostatic, ink jet, thermal printheads, and light emitting diodes. Because of their high resolution at reasonable cost, light emitting diodes (LED) printbars are frequently used.
In a typical light emitting diode printbar a large number of individual light emitting diodes are arranged in an elongated, planar array that is placed adjacent an image recording member. By providing relative motion between the light emitting diode printbar and the image recording member, the light emitting diode printbar scans the image recording member. By selectively illuminating the individual light emitting diodes and the image recording member is scanned a desired latent image can be recorded on the image recording member.
The selective illumination of the individual light emitting diodes is performed according to image-defining video data information which is applied to LED driver circuitry. Conventionally, the image-defining video data information took the form of simple binary video data signals. Those data signals might be from any of a number of data sources, such as a Raster Input Scanner (RIS), a computer, a word processor, or a facsimile machine. Typically, the binary video data was clocked into a shift register. After completely shifting the data into the shift register, the contents of the shift register was transferred in parallel into latch circuits for temporarily storage. Then, upon the occurrence of a start of line signal, the latched data was applied to LED drive circuits which illuminated the individual light emitting diodes of the light emitting diode printbar so as to produce a line of the latent image. A complete latent image was formed by performing successive line exposures until the complete late image was produced. U.S. Pat. Nos. 4,689,694; 4,706,130; 5,138,337 and 5,126,759 are representative of prior art printbar control circuitry. Prior art exposure control systems are disclosed in U.S. Pat. Nos. 4,525,729, 5,025,322 and in co-pending U.S. patent application Ser. No. 07/898,604 filed on Jun. 15, 1992. The contents of these prior art references are hereby incorporated by reference.
Prior art light emitting diode drive control-systems generally have limited individual light emitting diode drive control. The ability to specifically tailor the drive of each light emitting diode so as to optimize pixel-to-pixel uniformity, to enable multiple exposure levels (gray level exposures), or to accurately compensate for the electrical drive to optical output power differences between the individual light emitting diodes is adversely impacted by the architecture of the light emitting diode printbar drive control system. Therefore, an improved light emitting diode printbar drive control system would be beneficial.